In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, vanilla extract, granulated sugar, and salt until well combined.
Add the brioche bread cubes to the egg mixture, gently folding to ensure all pieces are soaked. Cover the bowl with plastic wrap and refrigerate overnight.
The next morning, preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking dish with butter or cooking spray. Pour the soaked bread mixture into the dish, spreading it evenly.
In a small bowl, mix the brown sugar and cinnamon. Sprinkle this mixture evenly over the top of the bread. Drizzle the melted butter over the entire dish.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the custard is set.
Let it cool for about 10 minutes before serving. Optionally, dust with powdered sugar and serve with fresh berries.